Gravity drop hammers are primarily designed for surface breaking of exposed rock.
These machines generally consist of a striker pin which extends outside of a nose cone which is positioned at the end of a housing that contains a heavy movable mass known as a monkey.
In a typical mode of operation, the weight of the machine is used to press the striker pin onto the surface to be broken. The correct positioning of the pin is known as priming which not only ensures the pin is at the right place, but is also in the striking position.
The movable mass is then lifted and allowed to drop onto the pin which then impacts the rock and the sequence is repeated until the rock breaks.
The striker pin wears away during use and is the main consumable tool of the breaking apparatus. Ensuring the pin is replaced at the optimal time is a key factor in cost-effective operation of the breaker. However, due to the cost and inconvenience of replacing the striker pin, there is a tendency amongst operators to continue to wear down the pin beyond the optimal replacement point.
It is an important operational requirement that the operator is able to sight the end of the pin. Reducing visibility of the pin tip reduces the operator's ability to locate the point correctly on seams or weak points. If the pin is not correctly primed onto rock, this can result in ‘dry hitting’, where only the nose block rather than the pin is resting on the rock. When a dry hit occurs, all the drop hammer energy must be absorbed into the hammer's buffer system and housing rather than the rock. Excessive dry hitting can cause structural damage and high wear and tear on parts increasing costs further. Moreover, a dry hit clearly does not break any rock, causing a consequential reduction in productivity. Typical drop hammers displaying such drawbacks are described in Australian Patent No. 585274.
The present invention throughout the specification will be discussed in relation to rock breaking apparatus invented by the applicant which is sold under the trade mark Terminator™. This will be understood to be exemplary only and the invention is not limited to use with same. The Terminator™ breaker represents an improvement (described in PCT Application No. PCT/NZ93/00074) over the hammer described in Australian Patent No. 585274.
The Terminator™ breaker is a gravity drop hammer that is configured for excavator carriers over 20 tonnes. Striker pins for this type of machine usually last around 500 hours and should be replaced after 25% of the pin is worn away. Replacing these pins costs around NZ$2,000 which represents 60% of the breaker operating costs. The Terminator™ breaker has design features (described in PCT Application No. PCT/NZ93/00074) to accommodate small numbers of dry hits and partial dry hits. However, it is still possible that excessive dry hitting can cause structural damage as described above in relation to other machines.
Even if the breaker is able to withstand dry hitting without sustaining damage, the adverse effect on productivity is still significant. As an illustration, the Terminator™ breaker typically produces 150 tons of rock per hour and the value of this material is around NZ$3 per tonne ($450 per hour). A 50% process loss or 75 tonnes/hour (which can be typical with dry hits) equates to NZ$225 per hour. The cost to run an excavator and Terminator™ breaker is around NZ$200 an hour irrespective of output, made up of labour, excavator costs, Terminator™ costs, fuel and so forth. This means that the operating loss is in the order of an additional NZ$100 plus the excess wear and tear caused by the dry hitting.
Therefore using the above figures, using an overly short striker pin can cost more than NZ$300 per hour. While it might seem obvious for the operator to change the pin earlier there are a number of factors dissuading them from doing so.
Breakers such as the Terminator™ breaker generally operate remotely from other plant and workshops and consequentially there is little equipment assistance to perform servicing work. Furthermore, it is impractical to return the breaker to the workshop for surfacing as it is semi-permanently attached to a digger. Detaching and subsequent re-attaching of the breaker and transportation to and from the workshop would typically require several hours.
A superficially simple solution is to increase the length of the pin extending out from the nose of the breaker so that it takes longer to wear down to an unusable size. However, such over-length pins are likely to snap during operation and thus this option is not preferred. The applicant has also devised a newer version of the Terminator™ described in the co-pending application NZ Pat App no. 543739 (referred to herein as Terminator II™) which, in addition to performing surface breaking tasks of conventional drop hammers, can also perform levering and high intensity raking. In contrast, conventional hammer manufacturers recommend against high intensity raking and levering due to the risk of shearing the striker pin.
Raking involves using the excavator to pull surface rock along the ground using the side of the pin. The rock can be loose above the ground surface or be friable enough to be drawn towards the excavator by pressing the point of the pin into the in-situ rock and dragging it across. Although the tractive resistance of the excavator does limit the maximum side forces applicable to the striker pin to a degree, the inertia of the two large pieces of equipment is high.
Levering is in particular a very useful action of the Terminator II™ rock breaker and involves driving the point of the striker pin into non-friable in-situ rock creating a crack. Once the crack is established, the operator can rotate the Terminator II™ at one end of the boom attached to the excavator to lever the rocks from the ground. Side forces are limited by inertia and the excavator hydraulics capabilities.
The operator can also use a hammer blow while levering to increase break out force. This is not feasible while raking so generally levering loads are a lot higher than raking loads but allow rock extraction in harder rocky deposits.
To perform these functions it is essential that a minimum pin length is maintained as raking is less effective with a short pin and levering is not possible at all.
It is thus desirable to provide a striker pin capable of an increased effective working lifespan, without breaking or requiring early replacement.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.
It is acknowledged that the term ‘comprise’ may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term ‘comprise’ shall have an inclusive meaning—i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term ‘comprised’ or ‘comprising’ is used in relation to one or more steps in a method or process.
It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.